An inveter is shown as a circuit which drives a multiphase AC load such as a motor. It is known that this inverter employs a configuration (referred to as a “capacitor-less inverter” below) which includes a matrix converter or include a DC link but not a smoothing circuit such as a larger capacitor or a reactor as a circuit system which achieves high efficiency and miniaturization.
Matrix converters include a direct matrix converter which is not provided with DC link and an indirect matrix converter which is provided with DC link. However, as introduced in Japanese Patent Application Laid-Open No. 2004-222338, even a direct matrix converter can be controlled based on an operation of a configuration in which a virtual AC-DC converter and a virtual DC-AC converter which are coupled through virtual DC link without a smoothing circuit.
Further, Japanese Patent No. 4067021 also introduces the above-described capacitor-less inverter, which has a capacitor in a DC link but the capacity of the capacitor is designed to be smaller than a capacity to function as a smoothing circuit. In this technique, voltage of the DC link is supposed to pulsate.
Hence, a circuit which converts AC power without a substantial smoothing circuit irrespectively of whether or not DC link are formally provided or whether or not a capacitor is provided is referred to as a direct AC-AC power converter in the present application.
Direct AC-AC power converter does not involve an energy buffer, and therefore harmonic components produced by a multiphase AC load are propagated to a power side, and harmonics of a power current increase. Reduction of the harmonics of the power current is demanded to prevent negative influences on surrounding environment. A specific example of this demand is, for example, a rule of IEC61000-3-2. According to this rule, harmonic components up to the 40th order with respect to a power frequency are regulated.
Japanese Patent Application Laid-Open No. 2005-27422 points out that, when, for example, a motor is used for a multiphase AC load and concentrated winding is adopted as a winding method of armature windings of the motor, a current (armature current) flowing in the armature windings includes harmonic components and, more particularly, the 5th order component and the 7th order component due to a differential voltage between a voltage outputted from an inverter and an inductive power of a rotating electrical machine. Such harmonic components (with respect to a fundamental wave component of a voltage outputted from the inverter) cause an increase in harmonics of a power current.
Japanese Patent No. 4488122 introduces a technique of reducing the 5th order component and the 7th order component.